1. Field of the Invention
The field of the present invention relates generally to control of engine operation for improved fuel economy, and in particular to control of an engine combusting a lean air-fuel ratio mixture in a first group of cylinders and operating a second group of cylinders without fuel injection.
2. Background of the Invention
Engine idle speed control is typically accomplished in stoichiometric engines by adjusting airflow to control engine speed to a desired engine speed. However, since airflow control may be slow due to manifold volume dynamics, ignition timing adjustment is also used. To allow for ignition timing adjustment to increase engine torque, nominal operation of idle speed control requires some basic amount of ignition timing retard. This basic amount of ignition timing retard, which is substantially present during all idle speed control, produces a negative impact on vehicle fuel economy since optimum torque (optimum ignition timing) is not always employed. Further, the range of authority using ignition timing alone is limited.
Another approach for controlling idle speed uses a lean burn engine. In this approach, fuel can be adjusted to quickly control engine idle speed. Here, ignition timing retard during normal engine auto speed control is not required to the extent as in stoichiometric engines, since adjustment of fuel injected can quickly change engine torque.
The inventors herein have recognized disadvantages with the above approach. In particular, since idle speed operation requires very low engine torque just enough to cancel friction and power accessories), the engine cylinders operate at a low torque level. Thus, when operating lean at these low torque levels, engine combustion stability is degraded. This degraded combustion stability can produce poor customer perception, and degraded speed control.
The above disadvantages of prior approaches are overcome by a method for controlling an engine having a first and second group of combustion chambers, the method comprising: inducting air and injecting fuel into the first combustion chamber group at a first average air-fuel ratio, inducting air and not injecting fuel into the second combustion chamber group, and adjusting the injected fuel to the first combustion chamber group based on a desired speed and a determined speed.
By operating some cylinder groups with air and fuel and others with air and substantially no injected fuel, the per-cylinder torque of the operating cylinders is increased while the overall engine torque to control engine idle speed is still the same. In this way, since the operating cylinder groups are operating at higher load, the cylinder groups can tolerate a lean air-fuel ratio with minimized degraded combustion.
As such, since the cylinders performing combustion have greater combustion stability, it is possible to operate the engine at leaner air-fuel ratios than heretofore possible. Furthermore, since larger ranges of air-fuel ratio are possible, the engine has a greater range of authority when using injected fuel as a feedback control variable to control engine speed. As such, not only is greater fuel economy provided, but also a greater engine torque control range is possible thereby resulting in better idle speed control.
Further, the combination of operating the engine at higher load and leaner air-fuel ratios, results in operation closer to open throttle thereby reducing engine pumping losses and increasing efficiency.
Also note that there are various methods for injecting fuel into the engine combustion chambers. For example, the engine can be a direct injection engine where a fuel injector provides fuel directly into the combustion chamber. Alternatively, the engine can be of an indirect injection type, where fuel injectors provide fuels into intake ports of an engine intake manifold coupled to the combustion chambers. Also, fuel can be in either a liquid or vapor, or combined form. Also, when operating a combustion chamber group at an average air-fuel ratio, this can mean that there is some variation between the actual air-fuel ratio combusted in the various combustion chambers. Additionally, this average air-fuel ratio can be varied according to a desired air-fuel ratio.
Further note that adjusting injected fuel based on a desired and determined speed can include torque based engine idle speed control. Here, the desired engine torque is determined based on a desired engine speed and a measured engine speed, via adjustment of injected fuel. Also, the desired torque can be adjusted to account for engagement and disengagement of accessories such as an air conditioner compressor. In addition, said adjusting of said injected fuel to the first combustion chamber group based on a desired and determined speed can include feed forward and feed back control.
Also note that said inducting of air into the combustion chamber can include inducting air and fuel (liquid or vapor).
Also note that the inventors herein contemplate and describe controlling determined speed to a desired speed in various forms, including: idle speed control, cruise control, an engine speed rev limiter, and stall prevention.